The present invention relates to an adjustable manhole top, and more particularly to a manhole top which can be raised up or down to correct problems caused when ground around the manhole heaves.
The ground about a manhole may heave, for example during winter conditions of consecutive frosts and thaws. This may make it very difficult for example for snowplows, the blades of which may then catch an upwardly projecting edge of the manhole cover or manhole cover support.
Conventional manholes have a foundation of circular cross-sectional shape, which may be of poured concrete. Above this foundation, and supported thereon, are bricks which in turn support a manhole cover plate having a manhole cover supporting rim. Asphalt or cement for the roadway is generally laid on or about the manhole cover plate. When the ground level about a manhole changes for whatever reason, for example from frost heaves or erosion of the ground about the manhole, repair work so that the manhole cover is again level with the surface of the roadway may require significant labour, including ripping up of the roadway surface and removal of the manhole cover plate and bricks, down to the concrete foundation. Thus, there is a need for a manhole top which can be easily adjusted, in a vertical direction, to take into account periodic changes in the level of the surrounding roadway.
Adjustable manhole tops are known. For example Canadian Patent No. 1,081,020 describes and illustrates a construction of manhole top in which screw bolts are passed through the manhole cover to bear against a support for raising or lowering the cover. Canadian Patent No. 1,161,263 describes and illustrates slip rings which go under the cover and its support to permit it being incrementally raised. Canadian Patent No. 68,905 describes and illustrates a construction in which the bottom of the manhole cover is provided with saw teeth on an incline to interact with corresponding saw teeth on an incline on a base support, to permit, upon relative rotation of the cover and base, the raising or lowering of the cover. Canadian Patent No. 1,068,961 features screw bolts which extend upwardly from the base to bear against the underside of the manhole cover to permit its raising or lowering. Such constructions possess many inherent difficulties, such as that of corrosion and seizing of the adjustment mechanisms or the need to rip up the roadway about the manhole top to activate the adjustment mechanism.
U.S. Pat. No. 2,254,668 of Tomek issued Sep. 2, 1941 describes and illustrates an upwardly and downwardly adjustable sleeve on which the manhole cover is supported. The sleeve telescopically fits on a cylindrical support base. The adjustable sleeve is levelled, to provide a proper manhole cover level, embedding a flange or the like which outwardly extends from the outer surface of the sleeve in the roadway surface while the sleeve is supported at a proper height. Again, this proposed construction faces many of the same problems of conventional manhole tops. The heaving of the roadway, for example, will result in breaking of the roadway about the manhole sleeve as a result of the flange embedded therein. This will require additional resurfacing of the roadway about the manhole sleeve. As well, the sleeve over time will tend to corrode and become seized in position on the cylindrical support base, making it extremely difficult to adjust.
Thus, it is an object of the present invention to provide a vertically adjustable manhole top which is designed to be easily vertically adjustable, with minimum need for excavation or resurfacing prior to, during or after adjustment. It is a further object of the present invention to provide such an adjustable manhole top which allows the passage of water, sand and the like through it, into the manhole, without clogging up the top. It is a further object of the present invention to provide such an adjustable manhole top in which corrosion does not significantly impede the adjustment operation.